Electronic devices to which an electric field is applied, such as organic electroluminescent elements (hereinafter, also referred to as “organic EL elements”), solar cells, and organic transistors, generally include charge-transporting thin films containing organic materials capable of transporting charge carriers (generic terms including electrons and holes) during application of electric fields. Various properties are required for functional organic materials contained in charge-transporting thin films, and such functional organic materials have been extensively developed in recent years.
Industrial components made of organic materials, particularly electronic devices and electronic components to which a high electric field is applied, generally involve problems due to the nature of the organic materials used therein, i.e. thermal degradation and electrochemical deterioration that are characteristics of organic substances. Most of technical improvements have been directed to enhance the robustness of such organic materials per se. For example, organic materials, such as copper phthalocyanine complexes, have been traditionally used in industrial applications. They are dyes, but have characteristics like those of pigments; they have a rigid chemical structure which renders them insoluble to any solvent, unlike other organic materials. The use of such organic materials unfortunately fails to exploit properties common organic materials have, such as solubility, flexibility, chemical reactivity, and compatibility with other materials. Such characteristics of organic materials are inappropriate for future industrial applications.
Organic materials are barely used in the form of a single isolated molecule in usual cases. They are often present in the form of an aggregate composed of a single type or different types of molecules (including different materials, such as metals or inorganic substances).
Meanwhile, molecular design has been essentially based on data of an isolated single molecule, typically data of structural analysis by X-ray diffractometry and molecular orbital calculation. In the actual circumstances, the way of molecular design has not been very active in view of the coexistence of multiple molecules. A technique has been therefore desired which focuses on the aggregates formed of molecules, to improve the stability of the aggregates at macroscopic level.
If no change occurs in a film or an article containing an organic material during preservation or use, its properties should not change in any way. Properties required for such a film or article may vary depending on its application field. Examples of the required properties include a specific color, charge transfer properties, and optical properties such as a specific refractive index. In any case, if the film or the article experience no change in its condition, its properties should not change in any way, that is, it should have infinite durability.
For example, charge-transporting thin films require continuous application of an electric field during use. Thus, they should have sufficient durability over time during application of current. In particular, they should not show unfavorable variations in charge transfer characteristics, i.e. variations in resistance, in view of their intended purpose of use. A charge-transporting thin film is therefore desired which shows a small change in resistance during the application of current.
Disclosed conventional techniques of improving the stability of the thin film rely on use of various compounds alone or in combination. For example, PTL 1 discloses a metal complex having a specific ligand as a blue phosphorescent compound. Some reports also disclose that combined use of two dopants which emit light of similar colors provide a device with higher efficiency, a prolonged lifetime, and a lower driving voltage (see, for example, PTLs 2, 3 and 4).
Unfortunately, even such techniques fail to achieve sufficient stability required for charge-transporting thin films, where a current is generally applied for a long time, under conditions expected in the market. There has been therefore need for a radical solution to such a problem.